To ensure traffic safety and to facilitate trade, design and construction aspects, vehicle headlamps are specified by internationally recognized UN/ECE regulations. Generally, it is of critical importance that the front beam profile satisfies strict requirements, for example requirements relating to the shape of a bright/dark cut-off line, the maximum light intensity that is allowed above the bright/dark cut-off line, etc. Other characteristics such as color temperature, lamp driver characteristics, lamp dimensions, etc. are also governed by applicable regulations. High-intensity discharge lamps (HID lamps) such as Xenon HID lamps are widely used in automotive headlamp applications, since they can provide an intensely bright light.
There are basically two different types of vehicle headlamps for use with a HID bulb, namely a projector headlamp and a reflector headlamp. In a projector headlamp a so-called S-type HID bulb is used, and in a reflector headlamp a so-called R-type HID bulb is used. In a projector headlamp, the light from the S-type HID bulb (e.g. a D2S bulb, a D4S bulb etc.) is focused on a shield by an elliptical mirror. The non-shielded light is projected by a lens into the final beam pattern. In a reflector headlamp, an R-type HID bulb is placed horizontally in an essentially parabolic reflector, and the reflector images the discharge arc outward and onto the road. To prevent glare, the R-type HID bulb (e.g. a D2R lamp, a D4R lamp, etc.) has dedicated black “pinstripes” on its outer glass vessel to prevent glare and to obtain the required cut-off, see e.g. US20070029916A1 and US20130063949A1. Additionally the reflector headlamp uses a metal shield placed in front of the R-type HID bulb in order to block any scattered light that would either leave the headlamp directly (i.e. without undergoing any reflection) or would be scattered by the HID bulb itself (i.e. by its salt composition) in an uncontrolled way into the reflector and then leave the lamp in the direction of oncoming traffic. The opaque stripes on the outer HID bulb surface generally comprise a ‘vertical’ stripe, i.e. a stripe arranged around the circumference of the lamp near the lamp base, and ‘horizontal’ stripes arranged along the length of the lamp, which is mounted essentially horizontally in a reflector of a lighting assembly, as described in EP 0 708 978 B1. The stripes serve to block stray light which would otherwise contribute to excessive glare in the beam pattern. An S-type lamp for use in a projector headlamp does not have any stripes, since the projector itself eliminates the stray light by an internal shield and ensures that the glare values are reduced to satisfy the legal beam requirements.
Because the known HID lamps are specifically designed as either R-type or S-type, it is not possible to use an R-type lamp in an headlamp designed for use with an S-type lamp, since an acceptable beam pattern cannot be achieved if an R-type HID lamp is inserted into an S-type headlamp, and vice versa. For example, an S-type bulb in an R-type headlamp would result in glare levels which are not acceptable. On the other hand, using an R-type bulb in an S-type headlamp would reduce the beam length and width by up to 50%, which is clearly also not acceptable.
The driver of an automobile cannot be assumed to know which type of lamp is allowed for the vehicle headlamp, and it may be that a driver is not sure which choice of bulb is correct. Choosing the correct bulb is also difficult in the face of great variety of bulbs being manufactured by the various suppliers. This makes the overall situation complicated and confusing from the point of view of manufacturer, retailer and consumer.
Currently there are solutions on the market which offer a HID bulb without coatings and a metal shield which can be assembled or disassembled as required, depending on whether the lamp is to be used in a projection headlamp requiring an S-type lamp, or in a reflector headlamp requiring an R-type lamp. The metal shield is intended to fulfill the same function as pinstripes applied directly to the lamp. Due to the sophisticated mounting construction, the required precision, the high temperature load and the large size of the metal frame, such solutions are very expensive to manufacture. Furthermore, the end user has to know which type of HID bulb he needs, and must then be able to attach or detach the metal frame, depending on his headlamp type. However, in this alternative solution, the metal shield cannot block unwanted light as effectively as pinstripes applied directly onto the lamp, and mounting or construction errors detract further from the quality of the resulting front beam.
Therefore, it is an object of the invention to provide an alternative HID lamp design that overcomes the problems mentioned above.